Are you talking about the 3 x 5 bags in the catty, or the super skinny ones that hold a stack of M&Ms, tall and skinny like a pencil.
For the 3x5, a great way to do it is with the top note die cut. Fold it one way for a wider bag topper, fold it the other way for a smaller bag topper. For the super super skinny ones I imagine you could use the curly label punch the same way, or many other punches. Scallop square, scallop circle, designer label, round tab punch etc. Then decorate with the bat punch, make a ghost with the owl punch (upside down and cut off the feet) etc. I don't do halloween, but there are lots of examples for decorating in the gallery, just search halloween. That's where I saw the ghost from the owl punch.